Forest monitoring has increasingly turned to satellites over the past several decades, and 2018 was no exception. In the last few months, NASA launched two sensors into space that will play a prominent role in monitoring forest biomass and structure over the next decade: the Global Ecosystem Dynamics Investigation (GEDI) now attached to the International Space Station, and the Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2).
From an article in Mongabay.
These two satellites, which in combination provide complete coverage of the planet, are equipped with lidar sensors that record forest structure in 3D, contributing to an ongoing wave of large-scale forest ecosystem measurements.
Researchers and forest managers working to record and reduce the rapid loss of forests are now armed with a new tool to monitor vegetation across the globe. In December 2018, a SpaceX rocket launched the NASA-engineered Global Ecosystem Dynamics Investigation (GEDI) sensor up to the International Space Station to gather data on the structure and extent of forests.
Scientists have increasingly relied on remote sensing methods to estimate the extent of forest landscapes, in particular collecting data from space at a large scale. Over the next several years, GEDI will provide the most accurate lidar (light detection and ranging) data on tropical and temperate forests ever to be collected from space.
Satellite lidar systems determine vegetation structure by emitting lasers down to Earth at a known distance from the planet’s surface and measuring the time it takes for the lasers to return to their origin. As the satellite orbits the Earth, its lasers bounce off different features of a landscape. Shorter return times correspond to taller features, such as the top of a forest canopy, while longer return times correspond to shorter features, such as grassy plains. Where elevation of an area is known, very precise heights of vegetation features can be determined across the landscape.
The benefit of GEDI’s lidar is its ability to collect forest structure data in three dimensions, which enables scientists to not just categorize different types of land cover but also record vegetation canopy height and tree density, which other satellite sensors cannot. GEDI’s lasers will also penetrate the forest canopy to map understory growth, surveillance that no satellite to date has been able to do and would otherwise be near impossible without the difficult fieldwork of measuring forest biomass and carbon storage from the ground.
Scientists agree that forests are disappearing rapidly and globally, but the speed with which the world is de-greening and how much of Earth’s carbon and biodiversity are lost along with forests are not well understood. The amount of carbon released when forests are cut down or burned depends on the amount of biomass the forests contain. Carbon emissions from deforestation play a substantial role in assessing the impact of human activity on climate.
“The largest gap is that we do not know the existing carbon stocks of the Earth’s forests,” Dr. Ralph Dubayah, the principal investigator of GEDI and professor of geography at the University of Maryland, told Mongabay. “Canopy height provides a direct link to a tree’s weight because just as with humans, larger trees weigh more than younger trees. About half a tree’s biomass is carbon. So if we know the heights globally, we can make much better maps of global forest carbon.”
Satellite-mounted lidar can also help scientists record how long it has been since a forest starting growing or was last cut down, also known as its successional state. Trees in old-growth forests tend to be bulkier and taller and therefore store more carbon and offer unique ecosystems that younger forests don’t.
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